In recent years, power supply systems utilizing distributed power supplies, such as gas engine generators and fuel cells, are drawing increasing attention. For example, there is one proposed system that uses a power generator called a distributed power supply mentioned above to supply electric power from the power generator to a plurality of electrical loads in a particular area (see Patent Literature 1 and Patent Literature 2, for example).
It is often the case that such a system, which is also called a microgrid, is configured such that a power system therein is connected not only to the aforementioned power generator but also to an energy storage that includes, for example, a secondary battery. The energy storage is provided for compensating for fluctuations in power generation by the power generator as well as load fluctuations. There is also a case where the power system is an AC power system, but the power generator is a DC power generator. In this case, a power converter for AC-DC power conversion is necessary. In such a system where a distributed power supply is utilized, self-sustained operation in which the system is operated independently of a commercial electrical grid, and interconnected operation in which the system is operated while being interconnected with a commercial electrical grid, are both taken into consideration. It is important to realize stable power supply in both of these operations. Also, the power converter is required to have functions that make such stable power supply possible.
In view of the above, the applicant of the present application has recently developed a technique of controlling a power converter that has characteristics equivalent to those of a grid-interconnected power generator (i.e., a power converter including a generator model). The applicant has proposed a power converter that realizes: stable power supply not only in the interconnected operation but also in the self-sustained operation; and uninterrupted transition from the interconnected operation to the self-sustained operation. Patent Literature 3 discloses a power converter whose control system incorporates, as a generator model, a differential equation called Park's equation in which the dynamics of a power generator are written. Patent Literature 4 also discloses a conventional power converter. In Patent Literature 4, a generator model is represented by a phasor diagram, and thereby the dynamics of a power generator are simplified. Such a simplified generator model is incorporated in a control system of the power converter.